Add drag-and-drop GUI for the V3 spectral bypass

Small Tkinter app: drop images in, get SynthID-cleaned copies out,
with an option to strip EXIF/XMP/IPTC metadata from the output.
Runs offline on the existing V3 codebook, no PyTorch required.

gui/README.md

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+# SynthID Cleaner (GUI)
+
+A small drag-and-drop desktop app around this repo's V3 spectral bypass
+(`src/extraction/synthid_bypass.py`). Useful for e.g. removing the SynthID
+watermark left behind after using generative AI to restore old photographs.
+
+- Drop images in, get cleaned copies out — no command line needed after setup.
+- Optionally strips all EXIF/XMP/IPTC metadata from the output (including any
+  AI-generation provenance tags such as C2PA content credentials or IPTC
+  `DigitalSourceType`), by rebuilding the file from raw pixel data.
+- Runs fully offline. No network calls, no telemetry.
+- Uses V3 (pure signal processing — numpy/scipy/opencv only, no PyTorch
+  required, no GPU needed).
+
+## Setup
+
+```bash
+cd gui
+python3 -m venv venv
+source venv/bin/activate
+pip install -r requirements-gui.txt
+```
+
+## Run
+
+Double-click `Launch SynthID Cleaner.command`, or from the terminal:
+
+```bash
+cd gui
+source venv/bin/activate
+python gui.py
+```
+
+The first double-click sets up the venv automatically if it doesn't exist yet.
+
+## Notes
+
+- Reads the codebook from `../artifacts/spectral_codebook_v3.npz` (already in
+  this repo) — no extra downloads.
+- Subject to this repo's [LICENSE](../LICENSE): non-commercial use, with
+  required attribution to the original author.